“At this stage, the only way we can reach an open convention is for Senator Cruz to be successful in as many of the remaining nominating elections as possible,” Romney said. He added “a vote for Governor Kasich in future contests makes it extremely likely that Trumpism would prevail.”

Romney campaigned with Kasich in Ohio before Tuesday's primary in Kasich's home state, which the governor won the state's primary, taking all 66 delegates.

Friday evening, Kasich's campaign released a web ad of video from Monday's rally in Ohio where Romney called said that "unlike the other people running, he has a real track record."

In an interview on MSNBC’s Hardball Friday, Kasich expressed disappointment in Romney’s vote.

“…so we just put one foot in front of the other and keep moving,” Kasich said. He added that he doesn’t think anyone will get to the Republican convention in July with enough delegates.

“We’ll ultimately have somebody chosen for the delegates who represent the Republican Party. Everybody just needs to calm down,” he said.